Sanders’s emerging strength has exposed continued misgivings among the party’s progressive base about Clinton, whose team is treading carefully in its public statements. Supporters have acknowledged privately the potential for Sanders to damage her — perhaps winning an early state or two — even if he can’t win the nomination.

“He’s connecting in a way that Hillary Clinton is not,” said Burt Cohen, a former New Hampshire state senator and Sanders supporter who attended Sunday morning’s event, where a nasty rain didn’t seem to deter many people from coming. “He’s talking about things people want to hear. People are used to candidates who are calculated, produced and measured, and they see through that. Bernie’s different.”